Since the PDP realizes higher definition and larger screen, product commercialization towards a television receiver of 65 inches class and a large public display device is advancing, and products exceeding even 100 inches are also being commercialized. In particular, the PDP for television receiver is advancing towards application to full spec high vision in which the number of scan lines is greater than or equal to twice of that of the conventional NTSC method.
The PDP is configured by a front plate and a rear plate. The front plate is formed by a glass substrate made of sodium borosilicate glass manufactured through float method, a display electrode including transparent electrode and bus electrode in stripe form formed on one of the main surfaces, a dielectric layer serving as a capacitor that covers the display electrode, and a protective layer made of magnesium oxide (MgO) formed on the dielectric layer. The rear plate is configured by a glass substrate, address electrodes in stripe form formed on one of the main surfaces thereof, a base dielectric layer that covers the address electrode, a barrier rib formed on the base dielectric layer, and phosphor layers formed between each barrier ribs to emit light of red, green, and blue.
The front plate and the rear plate have the respective electrode forming surface side facing each other, and the periphery air tightly sealed by a sealing material. The exhaust of a discharge space partitioned by the barrier rib and enclosure of discharge gas (in the case of Ne-Xe, pressure of 53.2 kPa to 79.8 kPa) are performed through exhaust pipe, which exhaust pipe is locally heated and melted (chip off) to be air tightly sealed after enclosing the discharge gas.
The finished PDP realizes color image display by selectively applying picture signal voltage to the display electrode to cause discharge, and exciting each phosphor layer with ultraviolet light generated by such discharge to emit light of red, green, and blue.
Low melting point frit glass having lead oxide as the main component is generally used for the dielectric layer of the PDP and the sealing material. The frit glass includes amorphous frit glass that does not crystallize when heated and in which the amorphous property still remains, and crystallized frit glass that crystallizes when heated. Each material has merits and demerits, and thus is selected in view of matching with the manufacturing step in most cases. When the frit glass of either the crystallized type or the amorphous type serves as the sealing material, filler is first mixed and kneaded with organic solvent to prepare sealing material in paste form. The sealing material is arranged at the periphery of at least one of the substrates of the front plate and the rear plate using film thickness printing, ink jet, or an application device equipped with a dispenser. Next, tentative firing is performed at a predetermined temperature at which the frit glass will not completely soften, so that the front plate and the rear plate are assembled while facing each other, and sealing is carried out at a sealing temperature higher than the temperature of tentative firing.
Use of no-lead material referred to as “lead free” or “leadless” that does not contain lead component even for the PDP is desired in view of recent environmental problems. An example of phosphoric acid (phosphoric acid-tin oxide etc.) sealing material and bismuth oxide sealing material that does not contain lead component is disclosed as the sealing material (see e.g., patent document 1, patent document 2 and the like). However, water resistance tends to be inferior in the sealing material having as the main component low melting point glass of phosphoric acid-tin oxide proposed as the sealing material of no-lead compared to the lead oxide sealing material used conventionally, and air tightness of the PDP cannot be substantially maintained. To this end, the bismuth oxide sealing material is getting attention as the no-lead material.
In the above-described manufacturing step of the PDP, the phosphor layers are fired in a phosphor baking oven immediately after the phosphor layers are formed on the rear plate. Thereafter, the sealing material is arranged on the peripheral edge of at least one of the substrates of the front plate and the rear plate, the sealing material of the substrate arranged with the sealing material is tentatively fired, and then the temperature is raised to the sealing temperature higher than the tentative firing temperature to soften (melt) the sealing material for air tight sealing. The phosphor layers are thus fired over a plurality of times.
The number of steps can be reduced and the steps can be simplified if the firing process immediately after the phosphor layers are formed on the rear plate is omitted and the phosphor layers are fired in the process of tentative firing and the sealing process of the sealing material.
Although the softening point temperature barely changes with respect to the heating temperature in the sealing material made of the conventional lead frit glass, the softening point temperature changes with respect to the heating temperature in the no-lead sealing material having bismuth oxide frit glass as the main component. Therefore, defects arise in the subsequent sealing if the sealing material is tentatively fired at the usual phosphor layer firing temperature.
[Patent document 1] Unexamined Japanese Patent Publication No. 2004-182584
[Patent document 2] Unexamined Japanese Patent Publication No. 2003-095697